I saw someone was using something like the card in the link below on there system for some networking gear on their pc. I am very curious what a person would need 8 serial ports for. What kind of stuff uses this?
Industrial and test equipment often uses serial ports. For example in the manufacturing facility where I work there are serial connections to meters, power supplies, signal generators, and the equipment under test. The test station must communicate with all these devices to run automated production tests.
SCADA Systems, which perform wide area data gathering and control, often communicate with field devices over serial connections, or using radios with serial connections.
Although Ethernet connections are becoming more common, there's still a lot of equipment with serial ports. Sometimes this is less expensive to make, sometimes it uses less power than LAN connections, sometimes the devices are older and LAN connections weren't common.
I work in a company that supplies a moving map and sensor management system for offshore systems like oil rigs and boats. We tend to use these kind of things to provide multiple inputs from GPS, AIS, output to dataloggers and alternative sites (where ethernet is not available) as well as input from our own proprietary transponders.
This was a few years ago, but we used to have a video conferencing system with multiple cameras and other devices. The cameras where motorized and you could control the pan/tilt/zoom of all the devices from the a single interface. The computer was connected to a touch screen display by through one of the serial ports, all the cameras where connected to the computer through the serial ports, the room lighting was controlled through the a serial port.
Another usage, several years ago, was for our dial-in modem pool for staff remote access before cable/dsl become common.. A Linux box had an eight port serial card, several of the ports where connected to an external modem.
We used them in the past (before the rise of the Internet) in our dial modem communications system to run banks of up to 64 modems on a PC (using QNX RTOS, not Windows), accepting incoming connections and providing remote emulation and file transfer capabilities for IBM mini computers. Multiport serial cards are very good at providing the throughput needed for this kind of application.